Untitled

<!DOCTYPE html>
<html>
    <head>
    </head>
    <body>
        <div id="post-wrapper">
            <?php include "post.php"; ?>
        </div>
    </body>
</html>

<script>
    $(document).ready(function (){
        setInterval(function(){
            $('#post-wrapper).load("post.php");
        },2000);
    });
</script>

#############
post.php
<?php

function getLikes($id_post) {
    global $conn; // pass $conn to function as $conn is out of function scope
    $query = "SELECT * FROM users WHERE id_user IN (SELECT id_user FROM likes WHERE id_post='{$id_post}' LIMIT 4");
    if ($result = $conn->query($query)) {
        if ($result->num_rows) {
            WHILE ($row = $result->fetch_assoc()) {
                $rows[] = $row;
            }
            return $rows;
        }
    }
    return null;
}

// your query
WHILE ($row = $result->fetch_assoc()) {
    $rows[] = $row;
}
?>
<?php foreach ($rows as $row): ?>
            <div class="post" id="<?= $row['postid']; ?>">
                <div class="content">
                    <div class="content-image"><?= $row['postimage']; ?></div>
                    <div class="content-text"><?= $row['content']; ?></div>
                </div>
                <div class="likes">
<?php if($likes = getLikes($row['id_post'])) {
                    echo '<ul>';
    for ($i = 1; $i <= 3; $i++) {
        // your logic
    }
                    echo '</ul>';
}
?>
                </div>
            </div>
<?php endforeach; ?>